1. Field of the Invention
The present invention relates to curable thermoplastic elastomeric blends, their manufacture and their use to make injection or extrusion molded rubber articles. More specifically but not by way of limitation, the present invention relates to blends of (i) a thermoplastic polyester (e.g., polyalkylene phthalates such as polyethylene terephthalate (PET) polybutylene terephthalate (PBT), related copolymers thereof and the like) or a thermoplastic polyester elastomer (e.g., polyether esters such as block copolymers consisting of segments of polybutylene terephthalate and segments of long-chain polyether glycols or the like) and (ii) a non-functionalized, cross-linkable, polyacrylate or polyethylene/acrylate vulcanizate rubber (e.g., a poly(meth)acrylate (ACM) type elastomer or a polyethylene/alkyl (meth)acrylate (AEM) type elastomer) in combination with a peroxide-free radical initiator and multiolefinic coagent for cross-linking.
2. Description of the Related Art
It is generally known in the art and an accepted commercial practice to employ curable polyacrylate elastomers to manufacture high-performance rubber parts having excellent resistance to lubricating oils and greases, which are therefore useful in selected automotive applications and the like. Typically, these manufacturing processes involve a gum rubber vulcanizate and a cross-linking curative system, which because of rheological properties require physical blending, compression molding and subsequent curing to make thermoset molded parts, wherein runners, sprues and other scrap are not recyclable, consequently driving up costs. Categorically the gum rubber vulcanizates are either polyacrylate elastomers (ACM) derived from copolymerization of acrylic acid ester monomers (e.g., ethyl, butyl, and methoxyethyl acrylate and can include some vinyl acetate), polyethylene/(meth)acrylate elastomers (AEM) derived from copolymerization of ethylene monomer and (meth)acrylic acid ester monomers (e.g. ethylene and methyl acrylate and can include other comonomers and grafts, see for example U.S. 2002/0004568 A1 incorporated herein by reference), or polyperfluoroalkyl acrylate elastomers (FPA) derived from polymerization of fluorinated acrylic ester monomer (for example, 1,1-dihydroperfluoro-n-butyl acrylate). It is also generally known that the polyacrylate elastomer can be functionalized by incorporating a relatively small amount of an additional comonomer such as an acrylate glycidyl ester, maleic acid or other comonomer having a reactive group present, including acid, hydroxyl, epoxy, isocyanate, amine, oxazoline, chloroacetate or diene reactive groups. These functionalized polyacrylate elastomers can then be advantageously cured using a curative coagent containing functional groups that covalently bond to the functionalized reactive sites of the polyacrylate elastomer.
One problem associated with the prior art curable polyacrylate elastomers is the inherent rheological limitations of high viscosity and low melt flow of their cured or partially cured state. Consequently, physical blending followed by compression molding and subsequent curing is usually necessary to achieve acceptable properties rather than extrusion or injection molding directly to a finished part (as discussed above). However, in European Patent 0 337 976 B1 and in U.S. Pat. No. 4,981,908 thermoplastic elastomer compositions are disclosed comprising blends of polyester resin (including segmented polyetherester elastomers commercially available under the trademark Hytrel® from E.I. du Pont de Nemours and Company) and dynamically vulcanized, covalently cross-linked acrylate rubber (including ethylene/methyl acrylate terpolymers containing about one mole percent of a carboxylic acid-containing comonomer commercially available under the trademark Vamac® from E.I. du Pont de Nemours and Company). The covalent cross-linking in these disclosures is achieved by employing a functionalized polyacrylate elastomer in combination with a reactive difunctional cross-linking agent. However, almost all of these difunctional cross-linking agents can also react with the ester units in the polyalkylene phthalate polyesters (i.e., an amine, hydroxyl or carboxylic acid group will exchange with the ester groups and epoxy or acid groups will add to the hydroxyl end groups), which leads to high viscosity and lack of reproducibility.
In an article entitled “Rubber-Thermoplastic Compositions. Part V. Selecting Polymers for Thermoplastic Vulcanizates”; A. Y. Coran, R. P. Patel, and D. Williams, Rubber Chemistry Technology, Volume 55, pages 116-136 (1982), approximately 100 thermoplastic vulcanizate compositions, based on 9 kinds of thermoplastic resin and 11 kinds of rubber, were prepared by dynamic vulcanization (i.e., rubber cured during mixing with molten plastic). At page 121 of the publication it is asserted that ethylene vinyl acetate (EVA) rubber must be peroxide cured and no sufficiently stable peroxide curative is known for use at the high temperature required for melt mixing polybutylene terephthalate (referred to as PTMT, polytetramethylene terephthalate). This article also disclosed a dynamic vulcanizate of PTMT and an ACM rubber (commercially available as Elaprin® AR153) that was a blend of an ethylacrylate/acrylic acid (99/1 parts by weight) copolymer rubber and 3 parts by weight of diglycidyl ether of bisphenol A cross-linking agent (see for example page 8 of EP0095919A2).
In contrast to the above mentioned dynamically vulcanized, thermoplastic elastomer composition based on a blend of a polyester, a functionalized acrylate rubber and a rubber phase cross-linking agent, GB 1 208 585 discloses and exemplifies melt-shapeable polymer compositions based on polyethylene terephthalate blended with an ethylene copolymer rubber (with or without cross-linking sites) and a polyisocyanate chain extending agent that in a preferred embodiment also cross-links the functionalized rubber.
In U.S. Pat. No. 4,275,180, a polymer composition comprising a cross-linked blend of an elastomer and a thermoplastic polymer substantially free of halogen is disclosed. Examples specifically include a functionalized ethylene/methyl acrylate terpolymer rubber (Vamac®) blended with segmented polyetherester copolymer (Hytrel®). This reference teaches cross-linking by any conventional method including irradiation or chemical cross-linking such as by the use of peroxide. However, all examples employ high intensity radiation (e.g., 12 Mrads) in the presence of a coagent (triallyl cyanurate). In a subsequently published patent application, EP 0 274 888 A1, a halogen-free, aluminum trihydrate-filled thermoplastic elastomer composition is taught. Comparative examples 1 and 2 specifically reference the ingredients and procedures of the examples in U.S. Pat. No. 4,275,180 and along with operative Example 3 employ 15 Mrads of ionizing radiation to cross-link a Hytrel®/Vamac® blend in the presence of triallyl cyanurate coagent. At page 7 of this reference it is stated that in some cases it may be desirable to add to the cross-linkable polymer composition a coagent to assist in the cross-linking reaction. It is then taught that such coagents usually contain multiple unsaturated groups that are believed to react with the initial radical formed on the polymer backbone to form a more stable radical, which undergoes a coupling reaction to form cross-links more readily than a chain scission reaction. Triallyl cyanurate is acknowledged as a coagent. In contrast to the irradiation induced cross-linking examples, all of the operative examples employing a peroxide curing agent (i.e., free-radical initiator) were free of triallyl cyanurate or other coagent.